The conversion process takes a snapshot of the old site at the moment of conversion, and brings those pages over to your new site.
If you subsequently make updates to the old site, you can re-convert individual pages.
Unfortunately, new pages added to the old site after the initial conversion are not a part of that initial snapshot and cannot be converted to the new site. You will have to recreate those pages in your new site.
For this reason, we highly recommend taking your new site online fairly quickly after conversion. You should focus efforts on building your new site, rather than making edits to the old site that will soon be replaced.